Download suspected carboniferous rocks of tavan har, gobi desert, mongolia

SUSPECTED CARBONIFEROUS ROCKS OF TAVAN HAR,
GOBI DESERT, MONGOLIA
MEGAN BERNEY ROBERTS
Whitman College
KHUUT TSETSEGMAA
Mongolia University of Science and Technology
Sponsor: Patrick Spencer
INTRODUCTION
The Zuun Gashuun Formation is located in the
Tavan Har area of the eastern Gobi Desert in
southeastern Mongolia. The main lithologies
of the formation include fossiliferous marble
(see figure 1), argillite, and quartzite. The
formation was originally mapped as
Ordovician, but recent macrofossil evidence
points to a younger, Carboniferous age.
The lithologies and fossils indicate that the
rocks were probably deposited on a low relief
microplate that was subject to oscillating
shallow carbonate seas. The microplate was
accreted during the late Paleozoic and early
Mesozoic as the North China block and
Siberian craton came together (Heubeck,
2001).
The goal of this study is to establish a more
precise age constraint on the rocks, to
correlate rock units, to gain an understanding
of the depositional environment of the rocks,
and to reconstruct the paleoenvironment and
relate it to the geologic history of Mongolia
and east-central Asia.
METHODS
Three stratigraphic sections of the Zuun
Gashuun Formation were measured and
described in order to construct a stratigraphic
column and correlate the rock units of the
formation. In addition, crinoid-rich marble
samples were collected in the field to be
processed for conodonts, in hopes of obtaining
a more precise age constraints. Lithologic and
macrofossil descriptions of hand samples were
used to provide further age constraints and
reconstruct the paleoenvironment.
AGE AND CORRELATION
The Zuun Gashuun Formation is currently
mapped as Ordovician in age. It was
originally hoped that conodonts would be
found to refine the dates of the supposed
Ordovician rocks. Unfortunately the only
conodont found was a small fragment of a
blade and can not be used for age refinement.
Figure 1: Fossiliferous, bedded marble in field
area.
However, macroscopic fossils indicate that the
Zuun Gashuun Formation is likely
Carboniferous in age. A brachiopod mold
displays features of a spiriferid brachiopod,
indicating that the rocks are either
Carboniferous or Permian. In addition, the
abundance and size (see figures 2 and 3) of
Figure 2: Some of the marble units consist of
a carpet of large crinoid stems.
Figure 3: Thin section of a well sorted,
crinoid-packed marble.
Figure 4: Stratigraphic columns and correlations of three traverses in the Zuun Gashuun Formation.
microplate. The units later experienced lowgrade metamorphism, which may have
occurred during or after the accretion of the
microplate. Many of the brachiopods and
bryozoans, which are more fragile than
crinoids, were probably broken due to wave
activity.
The marble protolith was deposited when sea
level was relatively high. The well-sorted
carbonate indicates that the deposition took
place in a shallow, tropical sea that was
subject to wave activity.
Figure 5: Papery argillite in field area.
crinoids in the marble suggests that the rocks
were deposited in the Carboniferous when
crinoids were more profuse. The large
crinoids, with stem segments up to
approximately two centimeters in diameter
(see figure 2), are too big to represent
Ordovician crinoids. The crinoid evolutionary
pattern of increasing size (Prothero, 1998)
raises the possibility that the rocks were more
likely deposited later in the Paleozoic.
Figure 4 shows three stratigraphic columns of
the Zuun Gashuun Formation. Each
stratigraphic column shows a large quartzite
unit followed by thick argillite that is often
interbedded with thin marble and quartzite
lenses. These quartzite and argillite units were
found in all three sections and are believed to
be correlative.
Most of the marble units are packed biospirites
that are well sorted and have abundant
crinoids. Brachiopods, bryozoans, and corals
are also present to a lesser extent. The
quartzite is silty, quartz sand with a slight
metamorphic fabric. The grains are angular
and well sorted. The argillite appears papery
in places (see figure 5), and occurs often with
thin lenses of marble or quartzite.
DEPOSITIONAL SETTING
The lithologic units of limestone, sandstone
and shale were deposited during transgression
and regression of the sea on a small
The argillite protolith was deposited when the
relative sea level was slightly lower. The
lenses of marble and quartzite represent
interplay between sediment supply and small
scale oscillations of relative sea level.
The quartzite protolith was deposited when
sea level was low, and represents near-shore
deposition. The quartzite was terrestrially
derived and was probably transported to the
sea by braided streams.
The frequent fluctuation of relative sea level
indicates that the microplate had low relief
that was influenced greatly by small changes
in sea level. The composition of the quartzite,
marble, and argillite suggests that the core of
the microplate was felsic and weathered to
quartz and clays.
During the time of the suspected deposition in
the Carboniferous, glaciers in Gondwanaland
were growing and melting, creating a large
scale oscillation of sea level. This may have
had an important influence on the deposition
of the rocks in the Zuun Gashuun Formation.
However, the stratigraphy of the formation
represents much more frequent fluctuations in
sea level than cyclothems show from the
Gondwanaland glaciation. Therefore, it is
possible that isostatic change in relief of the
microcontinent locally influenced relative sea
level. The isostatic change could be a result of
varying rates of conversion between the North
China block and the Siberian craton. Periodic
uplift of the microplate due to active
convergence would cause relative sea level to
drop. When the rate of uplift exceeded the
rate of glacial melt, sea level would appear to
drop. However, when the rate of uplift slowed
down or periodically stopped, sea level would
appear to rise. Such a phenomenon provides a
possible explanation for the numerous changes
in sea level exhibited by the stratigraphy of the
formation.
PALEOGEOGRAPHY
The core of Asia was assembled during many
terrane accretions during the late Paleozoic to
early Mesozoic (Heubeck, 2001). These
terranes were located in a paleo-ocean
between the converging Siberian Craton and
North China Block. The microplate on which
the Zuun Gashuun Formation was deposited
represents one of these terranes which was
accreted as the two blocks came together.
Lack of volcanic rocks in the measured section
indicates that the rocks were deposited before
the microplate was accreted. Although its
origin is unknown, the marble suggests that
the deposition took place near the equator in
warm and shallow seas.
DISCUSSION
The rocks of the Zuun Gashuun Formation,
previously thought to be Ordovician, are now
suspected to be Carboniferous. The rocks were
likely deposited during the Carboniferous in
shallow, equatorial seas. The stratigraphy of
the formation indicates that relative sea level
was oscillating quite frequently on the lowrelief microcontinent. During relative high
stands, fossiliferous limestone was deposited.
When relative sea level lowered, shale and
sandstones were deposited.
The oscillation of sea level may have been
caused by interaction between sea level
changes due to glaciation and relative sea level
changes due to an isostatic change in the
elevation of the microplate. It is suspected
that both of these processes were involved, as
the stratigraphy indicates frequent sea level
changes that could not be explained by merely
the growth and melting of glaciers.
Little research has been done in the Tavan Har
area, and more research is needed to better
understand the age, depositional processes,
and paleoenvironment of the Zuun Gashuun
Formation. In addition, further research
should be done in relating the Zuun Gashuun
Formations to the other formations in what is
believed to be the same terrane. The current
stratigraphic column of the terrane shows
other volcanic and clastic rocks as
representing the Carboniferous system. In
order to gain a more comprehensive
understanding of the geology of this area,
further research involving the whole terrane
must take place.
REFERENCES CITED
Heubeck, Chris., 2001. Assembly of central Asia
during the middle and later Paleozoic, in Hendrix,
M.S., and Davis, G. A., eds., Paleozoic and
Mesozoic tectonic evolution of central Asia: From
continental assembly to intracontinental
deformation: Boulder, Colorado, Geological
Society of America Memoir 194, p. 1-22.
Prothero, D. R., 1998. Bringing fossils to life: An
introduction to paleobiology: Boston, McGrawHill.